[unreadable] [unreadable] We request the final two years of funding to continue our work in Project 2 (Mechanisms of Transplantation Tolerance Resistance in Nod Mice) of the Program Project "Mechanisms of Islet Transplantation Tolerance" (PI Dr. Aldo Rossini). We request this funding to permit us to continue to 1) interact with other members of the Program, 2) access the cores that are essential for this project, and 3) keep the Overall Program intact as it comes up for competitive renewal. This Project proposes to investigate the genetic control of transplantation tolerance by costimulation blockade in NOD mice that are resistant to tolerance induction by costimulation blockade. To address this, we have developed new model systems to separate the mechanisms that control autoimmunity from those that control peripheral tolerance induction. The key observation is that the resistance of NOD mice to skin allograft tolerance, unlike its susceptibility to autoimmunity, is a genetically dominant (raft in NOD mice. In contrast, the resistance of NOD mice to islet allograft tolerance is a recessive trait and we have identified Idd loci that, in part, control this trait. We also document that we have developed unique TCR transgenic alloreactive chimeric model systems (synchimeras) to investigate the underlying mechanisms that lead to tolerance resistance in NOD mice. These new synchimeras are autoimmunity-free but express NOD-derived defects in transplantation tolerance and in the response of alloreactive CD4* and CDS* T cells to costimulation blockade. We propose to test the Overall Hypothesis that the mechanisms that control the induction of transplantation tolerance by Costimulation blockade differ from those that control the loss of self-tolerance and expression of autoimmunity. [unreadable] Specific Aim 1 is to identify the cellular basis for defects in tolerance induction in NOD mice. We will test the hypothesis that the dominant resistance of NOD mice to skin allograft tolerance induction is due to defects in the response of alloreactive CD4* or CDS* T cells, or in dendritic cells, to tolerance induction by costimulation blockade. Specific Aim 2 is to use our novel KB5 alloreactive CDS TCR synchimera model system to investigate the mechanisms by which CDS* T cells in NOD mice resist tolerance induction. We hypothesize that the relative resistance of NOD alloreactive CDS* Tce//s to deletion by costimulation blockade is a major barrier to tolerance induction. Specific Aim 3 is to use our novel D10 alloreactive CD4 TCR synchimera model system to investigate the mechanisms by which CD4+ T cells in NOD mice resist tolerance induction. We hypothesize that a second major barrier to the induction of tolerance in NOD mice is a defect in the response of alloreactive CD4+ T cells to co-stimulation blockade. The Specific Aims remain unchanged from the original submission 2 and one-half years ago. [unreadable] [unreadable] [unreadable]